The present invention relates to a new liquid, storage-stable coating composition, which is suitable for the production of coatings which have an outstanding resistance to external weathering, and binders thereof which are composed of a hardener component comprising an aminoplast resin and/or a blocked polyisocyanate and a specific polyol component of selected units, and to its use for the production of weathering-resistant coatings on any desired heat-resistant substrates.
One-component stoving lacquers based on polyisocyanates with blocked isocyanate groups and their use in polyurethane lacquers are known (cf. e.g. Kunststoff-Handbuch [Plastics Handbook], volume VII, Polyurethane [Polyurethanes], Carl Hanser-Verlag Munich (1966), pages 11-13, 21 et seq.).
For the use of such blocked polyisocyanates in combination with hydroxy-polyesters, there are the following main requirements:
1) Low splitting-off temperatures with the shortest possible stoving time.
2) No or only slight yellowing on stoving and brief over-stoving.
3) Adequate storage stability of the one-component lacquers.
4) No splitting off of toxicologically unacceptable cleavage products.
These requirements can be fulfilled by specific stoving binders based on hydroxy-polyesters and blocked polyisocyanates, such as are described in DE-A 2 550 156. It can be seen from DE-A 3 046 409 that the stoving binders of DE-A 2 550 156 cannot be used for the production of coatings which are exposed to shock and impact stresses in the surface region during protection of industrial goods, such as e.g. machine components, vehicle bodies or transportation containers.
Although the stoving binders of DE-A 3 046 409 give shock-and impact-resistant coatings, they cannot meet the normal demands of stability to weather. They are therefore unsuitable for coating surfaces which are exposed to extreme weathering influences, such as e.g. car bodies, facade elements etc., and for which high requirements are imposed on the mechanical properties and capacity for exposure to solvents, such as e.g. petrol.
One-component stoving binders based on hydroxy-polyesters and aminoplast resins also meet the requirements imposed after the stoving process only inadequately in respect of resistance to solvents and weathering, but lead to lacquer films which are distinguished by a favourable hardness/elasticity ratio and good adhesion. Such binder combinations are described in DE-A 2 621 657 and can be applied by the technique of automatic coil coating.
It can be seen from EP-A 494 442 that the stoving binders to date described above are unsuitable as stoving lacquers for intended uses where coatings are required of which the weathering properties must be so extremely high that in the pigmented state the lacquer films must also withstand short wavelength UV light for a relatively long time without yellowing and cracking and with high degrees of gloss of the lacquer film surfaces. Significant improvements in the UV resistance are shown by stoving lacquerings according to EP-A 494 442, where lacquer films are obtained which not only have a high level of hardness and elasticity with good adhesion to metal and good abrasion resistance but moreover withstand exposure to short wavelength UV light over a relatively long period of time without cracking and yellowing and without the addition of light stabilizers.
It is generally known in the lacquer branch that tests on lacquerings by means of intensive exposure to short wavelength UV light are not necessarily suitable for making exact predictions in respect of the resistance of lacquers to weathering in practice. Such accelerated laboratory tests with UV apparatuses sometimes do not correlate with long-term tests under realistic conditions in practice (e.g. in Florida), so that it may be that lacquers which show good weathering results in such laboratory tests give a poorer result in practice, thus also lacquers according to EP-A 494 442.
However, the use of metal sheets lacquered by means of automatic coil coating in areas where, in addition to a good resistance to solvents and chemicals, an excellent resistance to light and weather over a period of several years is required, e.g. on facades, is of increasing importance. Here also, a high level of properties is required in respect of the mechanical properties, such as e.g. hardness, elasticity and abrasion resistance.
Surprisingly, it has now been found that the coating compositions described below in more detail, based on a hardener component of the type known per se and a specific polyester component, give lacquer films which not only have a balanced hardness and elasticity ratio with good adhesion to metal and good abrasion resistance, but moreover show an exceptionally high, hitherto unknown resistance of such lacquer systems to external weathering in a test in practice lasting several years, in the form of an extremely high gloss retention, although accelerated weathering tests on these lacquers with exposure to UV light predict poor weathering properties.
The invention provides a liquid coating composition which is stable to storage at room temperature and comprises a binder mixture, organic solvents and optionally the conventional auxiliary substances and additives known from lacquer technology, where the binder is a mixture of
A) 2.0 to 30 wt. %, based on the sum of components A) and B), of a crosslinking component comprising at least one aminoplast resin and/or at least one blocked polyisocyanate
and
B) 98 to 70 wt. %, based on the sum of components A) and B), of a polyester component comprising at least one polyester containing hydroxyl-and carboxyl groups, characterized in that
component B) has an average number-average molecular weight Mn of 500 to 10,000 g/mol, a hydroxyl number of 20 to 50 mg KOH/g and an acid number of 0.5 to 30 mg KOH/g and has been prepared by polycondensation of
a) 50.5 to 53 mol % of a polyol component comprising
a1) 60 to 80 mol % neopentylglycol and/or 1,4-cyclohexanedimethanol,
a2) 20 to 30 mol % ethylene glycol, 1,2- and 1,3-propanediol, 1,3- and 1,4-butanediol and/or 1,6-hexanediol,
a3) 0.1 to 5 mol % trimethylolpropane, glycerol and/or pentaerythritol,
a4) 0 to 19.9 mol % of other alcohols which differ from a1) to a3) with
b) 47 to 49.5 mol % of a polycarboxylic acid component comprising
b1) 55 to 75 mol % isophthalic acid,
b2) 10 to 40 mol % terephthalic acid and/or phthalic acid or the anhydride thereof,
b3) 0.1 to 20 mol % adipic acid and
b4) 0 to 34.9 mol % of other carboxylic acids which differ from b1) to b3).
The invention also provides the use of this coating composition for the production of coatings which can be cured under the influence of heat on any desired heat-resistant substrates.
The binders of the coating composition according to the invention comprise 2 to 30, preferably 3 to 25 wt. % of a crosslinking component A) and 98 to 70, preferably 97 to 75 wt. % of a polyester component B), the percentages stated adding up to 100.
Crosslinking component A) comprises at least one aminoplast resin and/or at least one blocked polyisocyanate.
Melamine-formaldehyde or urea-formaldehyde condensation products, for example, are to be regarded as aminoplast resins. Suitable melamine resins are all the conventional melamine-formaldehyde condensates which are not etherified, partially etherified or completely etherified with saturated monoalcohols having 1 to 4 C atoms, such as are described e.g. in FR-A 943 411 or by D. H. Solomon, The Chemistry of Organic Film Formers, 235-240, John Wiley and Sons, Inc., New York, 1967. However, some or all of the melamine resins can also be replaced by other crosslinking aminoplasts, such as are described e.g. in xe2x80x9cMethoden der Organischen Chemie [Methods of Organic Chemistry]xe2x80x9d (Houben-Weyl), vol. 14/2, part 2, 4th ed. Georg Thieme Verlag, Stuttgart, 1963, 319 et seq.
Those blocked polyisocyanates which are based on aromatic polyisocyanates are less suitable as the blocked polyisocyanates, because of the extreme resistance to weathering required of the lacquerings according to the invention. Rather, those blocked polyisocyanates which are based on light-fast, aliphatic or cycloaliphatic polyisocyanates are possible according to the invention, such as e.g. 1,6-diisocyanatohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (iso-phorone-diisocyante), 1,3- and 1,4-cyclohexane-diisocyanate, tetramethylcyclo-hexane-1,3-and-1,4-diisocyanate, 4,4xe2x80x2-diisocyanatodicyclohexylmethane, 2,4xe2x80x2-diisocyanatodicyclohexylmethane etc. and mixtures thereof. However, chemical modifications prepared therefrom with preferably biuret, allophanate or urethane groups and uretdione and isocyanurate and/or iminooxadiazinedione structures can also be employed as crosslinking components after reversible blocking of the still free isocyanate groups. Such base compounds containing isocyanurate groups are described, for example, in the patent specifications EP-A 0 003 505, DE-A 1101 394, U.S. Pat. No. 3,358,010, U.S. Pat. No. 3,903,127, U.S. Pat. No. 4,324,879, U.S. Pat. No. 4,288,586, DE-A 3 100 262, DE-A 3 100 263, DE-A 3 033 860 and DE-A 3 144 672.
For the preparation of the reversibly blocked polyisocyanate component, the base isocyanates or mixtures thereof are preferably reacted with blocking agents such as e.g. xcex5-caprolactam, acetone oxime, butanone oxime, cyclohexanone oxime, 3,5-dimethyl pyrazole, 1,2,4-triazole diethyl malonate and diethyl acetoacetate, so that as a rule complete blocking of the free isocyanate groups is achieved.
The blocking reaction of the free isocyanate groups with xcex5-caprolactam or butanone oxime is carried out at temperatures between 100 to 130xc2x0 C., as described e.g. in DE-A 3 004 876.
The presence of catalysts, e.g. organotin compounds or particular tertiary amines, such as e.g. triethylamine, in amounts of between 0.01 to 0.1 wt. %, based on the total weight, is advantageous here.
The blocking reaction with malonic esters or acetoacetic acid esters takes place in a manner known per se (cf. DE-A 2 342 603 or 2 550 156) with the aid of basic catalysts, such as e.g. sodium phenolate, sodium methylate or other alkali metal alcoholates. Other organic alkali metal compounds, such as e.g. sodium malonate, are also possible. The catalysts are employed in an amount of 0.1% to 2%, based on the total weight of the reaction components. The amount of dialkyl malonate employed should be at least 1 mol per isocyanate equivalent; however, it is expedient to use a 5 to 20% excess of blocking agent.
In principle, it is also possible to only partly block a polyisocyanate which corresponds to definition A) but is unblocked, so that, for example, 40 to 90% of the isocyanate groups are present in blocked form, and then to react the partly blocked polyisocyanate with a polyol of the type suitable as component B). Analogously to this, the preparation of the combinations A) and B) according to the invention can be carried out not only as described below by mixing the individual components, but also by partly blocking unblocked polyisocyanates or polyisocyanate mixtures, so that, for example, up to 30% of the isocyanate groups are still present in the free form, and by then adding the polyester polyols suitable as component B) in an amount such that after the addition reaction which proceeds spontaneously between the free isocyanate groups and some of the hydroxyl groups of the polyester polyol, a mixture of blocked polyisocyanates and excess polyester polyol B) for which the equivalent ratio of blocked polyisocyanate groups to hydroxyl groups lies within the range essential to the invention of 0.6:1 to 2:1 is present. In such a case, component A) would comprise the said reaction product between the partly blocked polyisocyanate and the polyester polyol mentioned.
The blocking reaction can be carried out without a solvent or in the presence of a solvent or a mixture of solvents which is inert towards isocyanate groups. Possible such solvents are, for example:
esters, such as e.g. ethyl acetate, butyl acetate, methoxypropyl acetate, methylglycol acetate, ethylglycol acetate and diethylene glycol monomethyl ether acetate dimethyl esters or diethyl esters of adipic, glutaric and succinic acids; ketones, such as e.g. methyl ethyl ketone, methyl isobutyl ketone and methyl amyl ketone; aromatics, such as e.g. toluene and xylene, and the conventional higher-boiling hydrocarbon mixtures in lacquer chemistry.
The abovementioned solvents can be employed individually or as mixtures. Instead of solvents, it is also possible to use plasticizers, such as e.g. commercially available phosphoric acid esters, phthalic acid esters or sulfonic acid esters.
Blocked polyisocyanates which can be employed according to the invention as component A) are described e.g. in DE-A 2 342 603, 2 436 872, 2 550 156, 2 612 783, 2 612 784 or 2 612 785.
Mixtures of blocked polyisocyanates and aminoplast resins can also be used as component A) in the coating compositions according to the invention.
Component B) of the coating compositions according to the invention comprises polyesters with an average number-average molecular weight Mn determined by means of gel permeation chromatography (polystyrene calibration) of 500 to 10,000, preferably 1,000 to 8,000, particularly preferably 2,000 to 6,000 and very particularly preferably 3,000 to 4,500, with a heterogeneity H=(Mw/Mn)xe2x88x921 of  less than 5, preferably  less than 4 and particularly preferably  less than 3, a hydroxyl number of 20 to 50, preferably 25 to 46 and particularly preferably 30 to 42 mg KOH/g, and an acid number of 0.5 to 30, preferably 0.5 to 20 and particularly preferably 0.5 to 10 mg KOH/g, which have been prepared by polycondensation of
a) 50.5 to 53 mol % of a polyol component comprising:
a1) 60 to 80 mol % neopentylglycol and/or 1,4-cyclohexanedimethanol,
a2) 20 to 30 mol % ethylene glycol, 1,2- and 1,3-propanediol, 1,3- and 1,4-butanediol and/or 1,6-hexanediol,
a3) 0.1 to 5 mol % trimethylolpropane, glycerol and/or pentaerythritol and
a4) 0 to 19.9 mol % of other alcohols which differ from a1) to a3) with
b) 47 to 49.5 mol % of a polycarboxylic acid component comprising
b1) 55 to 75 mol % isophthalic acid,
b2) 10 to 40 mol % terephthalic acid and/or phthalic acid or the anhydride thereof,
b3) 0.1 to 20 mol % adipic acid and
b4) 0 to 34.9 mol % of other carboxylic acids which differ from b1) to b3).
The percentages of a) and b), a1) to a4) and b1) to b4) stated adding up to 100.
Esterification reactions can be greatly accelerated with the aid of catalysts. The polyesters B) are therefore preferably esterification products of:
a) 50.5 to 52.5 mol % of a polyol component comprising
a1) 62 to 78 mol % neopentylglycol and/or 1,4-cyclohexanedimethanol,
a2) 22 to 30 mol % ethylene glycol, 1,2- and 1,3-propanediol, 1,3- and 1,4-butanediol and/or 1,6-hexanediol,
a3) 1 to 4.5 mol % trimethylolpropane, glycerol and/or pentaerythritol and
a4) 0 to 15 mol % of other alcohols which differ from a1) to a3) with
b) 47.5 to 49.5 mol % of a polycarboxylic acid component comprising
b1) 57 to 73 mol % isophthalic acid,
b2) 12 to 38 mol % terephthalic acid and/or phthalic acid or the anhydride thereof,
b3) 2 to 18 mol % adipic acid and
b4) 0 to 29 mol % of other carboxylic acids which differ from b1) to b3),
which have been prepared using titanium- and/or tin-containing catalysts of a molecular weight range from 166 to 300. The percentages stated of a) and b), a1) to a4) and b1) to b4) adding up to 100.
The polyesters B) are particularly preferably esterification products of:
a) 50.5 to 52 mol % of a polyol component comprising
a1) 64 to 76 mol % neopentylglycol and/or 1,4-cyclohexanedimethanol,
a2) 24 to 30 mol % ethylene glycol, 1,2- and 1,3-propanediol, 1,3- and 1,4-butanediol and/or 1,6-hexanediol,
a3) 2 to 4.5 mol % trimethylolpropane, glycerol and/or pentaerythritol and
a4) 0 to 10 mol % of other alcohols which differ from a1) to a3) with
b) 48 to 49.5 mol % of a polycarboxylic acid component comprising
b1) 58 to 72 mol % isophthalic acid,
b2) 14 to 36 mol % terephthalic acid and/or phthalic acid or the anhydride thereof,
b3) 4 to 16 mol % adipic acid and
b4) 0 to 24 mol % of other carboxylic acids which differ from b1) to b3),
which have been prepared using tin-containing catalysts of a molecular weight range from 166 to 300. The percentages stated of a) and b), a1) to a4) and b1) to b4) adding up to 100
The polyesters B) are very particularly preferably esterification products of:
a) 51 to 52 mol % of a polyol component comprising
a1) 66 to 74 mol % neopentylglycol,
a2) 24 to 28 mol % 1,6-hexanediol,
a3) 2.5 to 4.5 mol % trimethylolpropane and
a4) 0 to 7.5 mol % of other alcohols which differ from a1) to a3) with
b) 48 to 49 mol % of a polycarboxylic acid component comprising
b1) 60 to 70 mol % isophthalic acid,
b2) 20 to 30 mol % phthalic acid or the anhydride thereof,
b3) 5 to 15 mol % adipic acid and
b4) 0 to 15 mol % of other carboxylic acids which differ from b1) to b3),
which have been prepared using tin-containing catalysts of a molecular weight range from 200 to 250. The percentages stated of a) and b), a1) to a4) and b1) to b4) adding up to 100.
The polyesters are prepared in a manner known per se by methods such as are described in detail, for example, in xe2x80x9cUllmanns Encyclopxc3xa4die der technischen Chemie [Ullmanns Encyclopaedia of Industrial Chemistryxe2x80x9d, Verlag Chemie Weinheim, 4th edition (1980), volume 19, pages 61 et seq. or H. Wagner and H. F. Sarx, xe2x80x9cLackkunstharze [Lacquer Synthetic Resins]xe2x80x9d, Carl Hanser Verlag, Munich (1971), pages 86 to 152.
The esterification is carried out in the presence of titanium and/or tin-containing catalysts of the molecular weight range from 166 to 300, preferably 200 to 250, at esterification temperatures of 80 to 260xc2x0 C., preferably 120 to 240xc2x0 C., and particularly preferably 160 to 220xc2x0 C. The esterification reaction is carried out until the required values for the hydroxyl and acid number are reached. After the condensation reaction, the polyester polyols are dissolved in suitable solvents, such as have been described above, for example, for the blocking of the polyisocyanates.
To prepare the binder combinations present in the coating compositions according to the invention with exclusive use of blocked polyisocyanates as component A), polyisocyanate component A) and polyester component B) are mixed, and in particular such that the ratios of amounts correspond to an equivalent ratio of blocked polyisocyanate groups to hydroxyl groups of 0.6:1 to 2:1, preferably 0.8:1 to 1.5:1, and particularly preferably 0.9:1 to 1.3:1. It should be ensured that mixing of the components is carried out below the temperature at which the blocked isocyanate groups can react with the hydroxyl groups.
As already mentioned, in addition to these essential binder components A) and B), the coating compositions according to the invention can also comprise auxiliary substances and additives or further binder components. These include, for example, other organic polyhydroxy compounds which are known from polyurethane lacquer technology and do not comply with the description of B), such as, for example, the conventional polyester, polyether or, preferably, polyacrylate polyols, i.e. copolymers, which are soluble in lacquer solvents of the type also employed according to the invention, of hydroxy-functional unsaturated monomers, such as e.g. hydroxyethyl and/or hydroxypropyl (meth)acrylate, with other olefinically unsaturated monomers, such as e.g. methyl methacrylate, styrene, acrylic acid, butyl acrylate, acrylonitrile or mixtures of such monomers. However, such polyhdyroxy compounds which do not comply with the definition of B) are at best used, if at all, in amounts of up to 50 hydroxyl equivalent %, based on the total amount of compounds containing hydroxyl groups which are present in the coating compositions. In the case where such polyhydroxy compounds are co-used, the amount of crosslinking component A) must of course be increased accordingly within the limits stated above, and in particular if blocked polyisocyanates are co-used as component A) the amount thereof must be such that the equivalent ratio of blocked polyisocyanates to hydroxyl groups is at least 0.6:1. The co-use of polyols which do not comply with the definition of B) is in no way preferred.
Further auxiliary substances and additives here are the conventional solvents with a boiling point of at least 75xc2x0 C. or a boiling range above 75xc2x0 C. The upper limit of the boiling point or boiling range of the solvents employed depends on the particular stoving conditions. The higher the stoving temperature, the higher the boiling temperatures of the solvents to be used should also be. Possible solvents are, for example, the following: aromatic hydrocarbons, such as e.g. toluene, xylene, tetralin and cumene, and technical-grade mixtures of aromatics with narrow boiling ranges, e.g. Solvesso(copyright) 100, 150 and 200 from Esso; ketones, such as e.g. methyl isobutyl ketone, diisobutyl ketone, isophorone and cyclohexanone, and esters, such as e.g. n-hexyl acetate, ethylglycol acetate, ethyl acetate, butyl acetate, methoxypropyl acetate or mixtures of such solvents.
The solvents can be added either, as already stated, during the preparation of the blocked polyisocyanates A), during the preparation of the polyester polyols B) or also at any desired later point in time.
Further auxiliary substances and additives which are optionally present in the coating compositions according to the invention are, for example, plasticizers of the type already mentioned by way of example, dispersing agents, thickeners, pigments, fillers, flow auxiliaries, light stabilizers, UV absorbers or catalysts which accelerate the crosslinking reaction.
As is known, the curing of polyester/aminoplast binders is accelerated by addition of acid. If polyesters with very low acid numbers are used, acid catalysts can be added to the coating compositions according to the invention. Thus e.g. the curing is greatly accelerated on addition of 0.5 wt. %, based on the sum of components A) and B), of p-toluenesulfonic acid.
It is also possible to add about 1 to 5 wt. %, based on the polyester, of an anhydride of a relatively acid dicarboxylic acid, e.g. maleic anhydride, to polyesters of low acid number in order subsequently to increase the acid number and in this way also to lower the stoving temperatures without the addition of further acid catalysts, and/or to shorten the stoving times.
The coating compositions according to the invention are liquid and storage-stable mixtures at room temperature and, as described above, can be formulated to processable lacquers by further additions.
To produce coatings using lacquers based on the coating compositions according to the invention, these are applied in one or more layers to any desired heat-resistant substrates by application methods known per se, for example by spraying, dipping, flooding or with the aid of rollers or doctor blades. The lacquers based on the coating compositions according to the invention are suitable for the production of coatings on metals, plastics, wood or glass. The lacquers based on the coating compositions according to the invention are particularly suitable for the production of coatings on aluminium and steel coils, which are coated by the automatic coil lacquering process and are used as vehicle bodies, housings for machines and domestic appliances, lining sheets, drums or containers. The substrates to be coated can be provided with suitable primer coats before the coating. In the use according to the invention, the amount of coating composition according to the invention employed is in general such that dry layer thicknesses of approx. 5 to 50 xcexcm are present. However, it is also possible to produce considerably thicker layers.
Curing of the lacquers based on the coating compositions according to the invention takes place in a temperature range of 90 to 500xc2x0 C., preferably 110 to 400xc2x0 C., and in a time from 45 to 0.25 minutes, preferably 35 to 0.50 minutes, depending on the use. The cured coatings have outstanding lacquer properties, and the exceptionally good resistance to external weathering, above all also under specific climatic conditions (e.g. Florida, USA), is to emphasized in particular.
The surface protection which can be achieved according to the invention is demonstrated in the following examples. All the percentages relate to percentages by weight.